The present invention relates generally to the field of magnetic resonance imaging. More particularly, the invention relates to a technique for providing magnetic resonance echo shifting without the need for shifting the radio-frequency pulse, or the data acquisition window, or both, such as for producing water-only or fat-only images, in a highly time efficient manner through a Dixon imaging pulse sequence.
Magnetic resonance imaging (MRI) systems have become ubiquitous in the field of medical diagnostics. Over the two past decades, improved techniques for MRI examinations have been developed that now permit very high-quality images to be produced in a relatively short time. As a result, diagnostic images with varying degrees of resolution are available to the radiologist that can be adapted to particular diagnostic applications.
In general, MRI examinations are based on the interactions among a primary magnetic field, a radiofrequency (rf) magnetic field and time varying magnetic gradient fields with nuclear spins within the subject of interest. Specific nuclear components, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of spins of such nuclear components can be influenced by manipulation of the fields to produce rf signals that can be detected, processed, and used to reconstruct a useful image.
The magnetic fields used to generate images in MRI systems include a highly uniform, static magnetic field that is produced by a primary magnet. A series of gradient fields are produced by a set of three gradient coils disposed around the subject. The gradient fields encode positions of individual volume elements or voxels in three dimensions. An rf coil is employed to produce an rf magnetic field. This rf magnetic field perturbs the spin system from its equilibrium direction, causing the spins to precess around the axis of their equilibrium magnetization. During this precession, rf fields are emitted by the spins and are detected by either the same transmitting rf coil, or by a separate receive-only coil. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one of several possible reconstruction algorithms to reconstruct a useful image.
Many specific techniques have been developed to acquire MR images for a variety of applications. One major difference among these techniques is in the way gradient pulses and rf pulses are used to manipulate the spin systems to yield different image contrasts, signal-to-noise ratios, and resolutions. Graphically, such techniques are illustrated as “pulse sequences” in which the pulses are represented, along with temporal relationships among them. In recent years, pulse sequences have been developed which permit extremely rapid acquisition of large amounts of raw data. Such pulse sequences permit significant reduction in the time required to perform the examinations. Time reductions are particularly important for acquiring high resolution images, as well as for suppressing motion effects and reducing the discomfort of patients in the examination process.
Among the pulse sequences which have been developed for fast acquisition of large amounts of MR data, is a sequence generally referred to as fast spin echo (FSE). This technique is capable of generating high-quality image data in a fraction of the time needed for conventional spin echo imaging. FSE techniques have thus become the sequence of choice, especially for T2-weighted imaging. However, a prominently distinguishing feature of FSE images is an anomalously bright signal resulting from fat content in the tissue being imaged. The phenomenon has been attributed to the demodulation of the J-coupling and de-sensitization of diffusion through inhomogeneities due to the rapidly refocusing rf pulse trains contained in the FSE pulse sequence.
Fat suppression has therefore become desirable in T2-weighted imaging procedures. At present, several techniques have been employed for such fat suppression. A first such technique is referred to as chemical saturation, and can be used to reduce the fat signal, but requires very homogeneous magnetic fields due to the close separation of the water and fat signals resulting from the excitation. In particular, the rf pulse must saturate all fat, requiring a highly uniform main magnetic field, to avoid separating water signals. Similarly, the technique depends highly upon the homogeneity of the rf field which is needed to achieve an accurate flip of the fat signal for suppression and subsequent flip of the resulting water signals for imaging. Inhomogeneity in the main magnetic field is particularly a problem at locations off the isocenter of the field system. Finally, patient anatomy also tends to perturb the fields, rendering the technique particularly problematic.
A second technique that has been developed for fat suppression involves short inversion time (TI) inversion recovery, and is commonly referred to as STIR. This technique is intended to flip all signals to an inverted direction, with fat and water signals recovering at different rates. The technique then acquires the image data when the fat signal is crossing the null point while the water signal is still partially in the inverted state. Because of its underlying principles, the technique typically is dependent on the T1 of the water signal, and generally results in relatively low signal-to-noise ratios due to the partial recovery of the water signal during the recovery of the fat signal.
A further technique that has been developed is generally referred to as the Dixon technique. In this approach, the chemical shift difference between water and fat is encoded into images with different echo shifts. Field inhomogeneity effects appear as image phase errors, which in principle can be corrected for by a combination of multipoint acquisition and more elaborate image processing. While these techniques allow for more uniform water and fat separation in the presence of field inhomogeneity, one clear drawback is the requirement for multiple data acquisitions and therefore longer scan times.
Incorporating the Dixon approach with fat suppression into FSE pulse sequences presents a mutually beneficial combination. While the Dixon technique provides a potentially robust separation of the strong fat signal, FSE helps to alleviate for long data acquisition times in the multipoint Dixon technique. In an exemplary combination of these techniques, however, echo shift as dictated by the Dixon technique was achieved by shifting the timing of the readout gradient and the data acquisition window to maintain necessary conditions (Carr-Purcell-Meiboom-Gill; “CPMG” conditions). As a result, inter-echo spacing was increased, leading to substantial loss in the slice coverage for a given sequence repetition time, largely offsetting the gain of using FSE for reducing the scan time. The technique is believed, therefore, to be appropriate for imaging small anatomic areas only that do not require large slice coverage.
Dixon technique based on the conventional spin echo or gradient echo sequences generally employ shifting the echo through either shifting the RF pulse, or the data acquisition window, or both. As in the case of FSE based Dixon technique, such shifting may lead to the disadvantage of longer acquisition times because of the increased deadtime during a sequence. Consequently, the loss of slice coverage for a given span time, or increased scan time for a given number of slices, and an increase in blurring and greater sensitivity to flow and motion artifacts, can all result.
There is a need, therefore, for an improved technique for obtaining shifts in echos in MR imaging sequences. There is a particular need for a FSE-based Dixon imaging approach which achieves the echo shifts satisfying the CPMG conditions without necessitating an increase in echo spacing. There is, at present, a particular need for an improved technique which can be implemented on existing hardware and control systems to obtain the improvement in timing and imaging clarity in a relatively straightforward manner.